Dynamic melting of proterozoic upper mantle - Springer Link

Contributions to Mineralogy and Petrology

Contrib. Mineral Petrol. 72, 165-173 (1980)

9 by Springer-Verlag 1980

Dynamic Melting of Proterozoic Upper Mantle: Evidence From Rare Earth Elements in Oceanic Crust of Eastern Newfoundland D.F. Strong ~ and J. Dostal 2 t Department of Geology, Memorial University of Newfoundland, St. John's, Newfoundland, A1B 3X5, Canada 2 Department of Geology, St. Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada

Abstract. Rare Earth Element (REE) data confirm earlier suggestions from major and trace elements that the Proterozoic Burin G r o u p in the southwestern Avalon Zone is similar to modern oceanic tholeiites, and also exhibit a systematic evolution through the sequence. The G r o u p forms a 60-km long belt consisting of four formations of pillowed basalt, two of subaqueous volcaniclastic and minor stromatolitic sediments and pyroclastics, and a thick gabbro-quartz diorite sill, with a total thickness of about 5000 m. Basalts of the oldest formation are enriched in light (LREE) with a chondrite-normalized pattern similar to alkali basalts. REE patterns through the rest of the sequence can be matched by those of modern ocean basins, with a steady decrease in total R E E and a distinct depletion in L R E E at the top of the sequence. R E E patterns of the gabbroic sill are similar to those of oceanic gabbros, with L R E E depletion and a small positive Eu anomaly. This evolutionary pattern can be interpreted as the result of progressive ' d y n a m i c ' partial melting and depletion of a single mantle source region.

earth elements (REE) distribution patterns. Although each model appears to satisfy the data for which it was devised, only dynamic melting, incorporating aspects of all the others, appears to be generally applicable (Langmuir et al. 1977). Nevertheless, even this model remains to be tested against field data which are not available for isolated dredge samples or tectonically disrupted ophiolites. Such an opportunity is provided by the Burin Group, a Proterozoic ( ~ 7 6 0 Ma., T.P. Krogh, personal communication 1978) assemblage of basaltic composition exposed in southeastern Newfoundland (Strong et al. 1978). Because formations of the G r o u p show a systematic chemical evolution with time (i.e., with stratigraphic position), we decided to determine if R E E patterns show any systematic variations predictable by partial melting models, thus allowing a test of such models against the filed observations. It was also of interest to investigate whether Proterozoic magmatic processes and products were comparable to those of the Phanerozoic, since changes in patterns of Precambrian-Phanerozoic crustal evolution have been suggested by a number of authors (e.g., Wynne-Edwards 1976).

Introduction Geological Setting Numerous attempts have been made to model processes of mantle melting through studies of Phanerozoic ophiolites (e.g., Malpas 1978; Menzies et al., 1977) and modern oceanic crust (Hanson 1977; Hanson and Langmuir 1978; Langmuir et al. 1977; Pearce and Flower 1977; Pankhurst 1977; O ' H a r a 1977). These models generally consider variations in degree of melting of ascending mantle (diapirs) and also the proportion and mechanisms of extraction of melt from the rising diapir. Processes which are considered, e.g., zone refining, fractional melting, batch melting, and dynamic melting, are dominantly based on rare

The Burin Group occurs in the Avalon Zone of southeastern Newfoundland (Fig. 1), a terrain dominated by Proterozoic volcanic rocks of bimodal basalt-rhyolite composition deposited in a subaerial environment (Strong et al., 1978). Rocks correlative with the Avalon Zone extend along the eastern edge of the Appalachians and parts of western Europe and Northwest Africa (Rast et al. 1976 ; Strong 1979). The Burin Group rocks differ from most volcanic rocks of the Avalon Zone in having a submarine origin and dominantly oceanic tholeiite composition. There may be numerous other ophiolite-like correlatives in the Avalon terrains of the north Atlantic region but the 762 Ma-old ophiolite of Bou Azzer in Morocco (LeBlanc 1975) is perhaps the closest analogy. There are also many other Proterozoic ophiolite terrains of Africa

0010-7999/80/0072/0165/$01.80

166

D.F. Strong and J. Dostal: Dynamic Melting of Proterozoic Upper Mantle

N

Gobbroic to granitic intrusive rocks Eocambrion to Cambrian sedimentary rocks Marystown Group subaerial volcanic rocks Burin

V

Group

e•v v

v

v

g

v

Grand Grand

Bea~

BURIN GROUP /v

v

v

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Beaver Pond Formation

Burin

"

Wandsworth (gabbro) Formation Path End Formation St, Lawrence

Lamaline

I'o

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Port ou Bras Formation

[~l

Pardy Island Formation

20 km Rock Harbour Group

Fig. 1. Geological setting of the Burin Group (after Strong et al. 1978)

which are just beginning to receive detailed geochemical study (e.g., Bakor et al. 1976). The Burin Groups is a steeply dipping volcanic sequence extending 60 km along a strike, unconformably overlain by Eocambrian strata. The Group has been divided into five formations, three predominantly of pillow basalts, one a gabbroic sill, and one of waterlain basaltic pyroclastic rocks. With decreasing age, the formations are: Pardy Island, Port au Bras, Path End, Beaver Pond and Wandsworth (Gabbro). The Wandsworth Formation is a gabbroic sill intruded along the boundaries between the Path End and Beaver Pond Formations. The Pardy Island Formation averages 2 km in thickness and consits mainly of pillowed basalt with minor tuffaceous and argillaceous sedimentary rocks. The pillows are generally aphyric, with calcite amygdules. The Port au Bras Formation is composed of fine-grained waterlain mafic tuff with minor limestone lenses and breccia, agglomerate and pillowed basalt. It has a maximum thickness of about 2 km. The Path End Formation is predominantly pillowed basalt with minor waterlain mafic pyroclastic rocks that are comparable to those of the Port au Bras Formation. It reaches a maximum thickness of approximately 1.5 km. The pillow lavas of these two formations differ from those of the underlying Pardy Island Formation in that they are rich in augite and plagioclase phenocrysts and they are consistently epidotized, chloritized or silicified. The Wandsworth Formation comprises a thick gabbroic sill complex, occupying an average stratigraphic thickness of about 1.5 km over a distance of more than 25 km, showing intrusive relations with both the underlying Path End and the overlying Beaver Pond Formations. It ranges from coarse-grained cumulusbanded zones to finer grained chilled margins and apophyses, and

contains large volumes of mutually intrusive diabasic and gabbroic dykes and abundant sheet-like xenoliths of the basaltic country rocks. The Beaver Pond Formation, the uppermost unit of the Burin Group, has a maximum thickness of 1.5 km. It is composed of pillowed basalt, which differs from that of the underlying formations in the large pillow size (averaging 2 3 m across), their lack of phenocrysts or amygdules, and a reddish-brown weathering colour.

Petrography The pillow lavas of the Pardy Island Formation are commonly porphyritic, with up to 20% each of titaniferous augite and plagioclase phenocrysts, and rare olivine phenocrysts. The rocks are generally altered, with groundmass minerals replaced by chlorite, actinolite, albite, calcite, epidote, quartz, and hematite; the phenocrysts are variably altered, with olivine pseudomorphed by serpentine, plagioclase partially replaced by albite, epidote and sericite, and clinopyroxenes (the least altered) by chlorite and actinolite. Both samples of the Pardy Island Formation analysed for REE (Table 1) were obtained from pillowed basalt, incorporating the pillow rims and about 10 cm towards the pillow cores. Sample T238B is ankaramitic, with titanaugite and plagioclase phenocrysts, and minor olivine microphenocrysts, all of which are altered as described above. Sample T289 is non-porphyritic, consisting only of a submicroscopic intergrowth of chlorite, albite and opaque oxides with veinlets of calcite, probably derived from basaltic glass. The pillow lavas of the Port au Bras Formation generally contain quartz and/or calcite amygdales, may be rich in clinopyrox-

D.F. Strong and J. Dostal: D y n a m i c Melting of Proterozoic Upper Mantle

167

Table 1. Chemical composition of Burin G r o u p rocks analysed for REE. Note that major element oxides have been re-calculated to 100% volatile-free, but original totals are given. Trace elements and REE in p p m Oxides

Pardy Island

Port au Bras

Path End

T-238B

T-289

W-083A

A-038C

T-069D

SiO2 TiO2 A120 3 F%O3 FeO MnO MgO CaO Na20 K20 P205 L.O.I.

54.35 0.75 15.42 1.33 6.65 0.17 7.76 7.90 4.09 1.40 0.09 2.68

51.17 2.15 15.34 3.19 10.23 0.21 6.41 5.89 4.58 0.64 O.14 6.16

49.34 1.47 17.13 6.21 8.98 0.28 5.68 5.45 3.69 0.69 0.41 4.50

46.22 1.69 19.68 2.68 9.69 0.26 5.29 8.84 2.95 2.25 0.42 11.03

Total

99.18

99.34

100.91

Zr Sr Rb Zn Cu Ba Th Nb Ga Pb Sc Ni Cr Co V Y

85 584 29 73 72 1088 3 2 18 0 30 64 230 51 193 18

150 197 8 126 76 318 0 5 21 6 39 29 61 45 345 24

65 187 11 130 151 221 0 6 21 7 34 9 11 58 343 22

Beaver Pond

Wandsworth Gabbro

T-195

T-208

T-390

T-141

T-383

T-190B

55.23 0.94 19.68 2.35 4.98 0.12 5.28 4.53 5.86 0.72 0.22 3.71

56.54 0.50 14.45 2.74 4.58 0.17 6.12 12.41 2.36 0.03 0.00 3.40

52.15 0.93 14.09 2.48 9.32 0.23 8.09 9.74 2.87 0.07 0.00 3.45

50.70 1.16 15.01 6.98 4.49 0.20 5.40 13.11 2.82 0.09 0.00 2.18

51.72 0.35 17.34 1.45 5.99 0.16 9.60 11.15 2.16 0.01 0.00 4.53

51.44 0.95 15.28 3.16 8.14 0.21 8.01 9.56 3.14 0.06 0.00 3.47

43.53 1.48 14.16 10.11 9.81 0.23 7.68 11.19 1.64 0.12 0.00 3.49

99.90

98.14

98.81

100.63

99.39

100.76

99.64

99.46

101 187 61 122 151 221 0 11 21 7 28 9 24 47 397 22

135 256 19 71 25 224 5 7 14 1 19 19 100 28 204 18

26 107 1 55 80 32 3 5 10 2 31 94 380 46 214 10

50 95 1 89 64 36 1 4 18 6 46 69 135 58 280 21

38 159 5 105 473 52 0 4 15 8 52 72 37 79 976 14

38 28 1 78 86 30 1 4 11 9 46 46 101 52 314 15

68 104 3 74 83 24 2 5 14 6 45 68 154 55 275 19

21 89 3 50 65 25 3 5 14 3 47 95 241 48 199 11

REE La Ce Nd Sm Eu Tb Yb Lu

11.2 24.3 13.2 3.48 1.04 0.479 1.55 0.256

13.5 30.7 18.6 4.33 1.46 0.568 1.72 0.286

4.54 12.5 9.92 3.07 1.17 0.779 3.22 0.512

6.63 18.0 4.16 1.61 1.01 4.03 0.634

3.76 ll.2 2.68 0.986 0.686 2.34 0.390

ene with some plagioclase phenocrysts, and are generally chloritized or silicified, with a b u n d a n t epidote and quartz veins. The samples used for REE determinations, WO83A and A038C (Table I), are both thoroughly altered to chlorite, albite, epidote, quartz, calcite, magnetite, and pyrite. Sample A038C is relatively coarse-grained, with about 5% altered plagioclase phenocrysts. The pillow lavas and pyroclastics of the Path End F o r m a t i o n are generally comparable to those of the Port au Bras F o r m a t i o n in terms of alteration. However, they are more c o m m o n l y porphyritic, with plagioclase phenocrysts being more a b u n d a n t than those of clinopyroxene, and opaque minerals are rare. Sample TO69D (Table 1) is a highly altered devitrified basaltic glass with rare plagioclase phenocrysts, and dominated by actinolite and epidote with a b u n d a n t quartz and calcite veinlets. Sample T195, from the chilled rim of a pillow, is highly vesicular, with amygdales of

1.64 4.47 3.13 0.952 0.311 0.229 0.993 0.171

0.840 2.45 3.94 1.37 0.539 0.474 2.39 0.413

1.82 6.52 2.60 1.04 0.657 3.12 0.592

0.214 0.704 1.10 0.690 0.327 0.209 1.22 0.227

1.05 3.42 4.45 2.13 0.897 0.616 3.04 0.607

0.597 1.97 1.33 0.583 0.358 2.16 0.421

quartz, epidote and calcite and a groundmass of albite laths, epidote, chlorite, quartz and magnetite. The pillow lavas of the Beaver Pond F o r m a t i o n are non-porphyritic, although minor plagioclase and clinopyroxene microphenocrysts are seen locally. They are generally altered to variable proportions of chlorite, epidote, actinolite, pumpellyite, calcite, quartz, sphene, zoisite, and stilpnomelane. The less altered varieties display quenched intergrowths of acicular augite, plagioclase and opaques. Sample T208 (Table 1), obtained from a pillow rim, is an altered glass with amygdales of epidote, veinlets of quartz and epidote, and about 2 % clinopyroxene microphenocrysts clots. Sample 7"390 is similar to T208 except that plagioclase and olivine microphenocrysts are present instead of clinopyroxene, and calcite veins occur as well as quartz and epidote. The Wandsworth Gabbro is c o m m o n l y coarse-grained,

168


equigranular, and dominated by variable proportions of plagioclase and clinopyroxene, ranging from anorthosite to clinopyroxenite in some cumulate zones, with minor amounts of magmatic hornblende. The mafic minerals are mainly altered to actinolite, and less commonly to chlorite, calcite and epidote, and plagioclase to sericite, calcite and epidote. Sample T141 (Table 1) contains large poikilitic actinolitized clinopyroxene crystals enclosing euhedral plagioclase, with minor primary hornblende and magnetite. The plagioclase is partially replaced by zoisite and hydrogrossular. Sample T383 exhibits a fine-grained chloritized diabasic matrix with relatively unaltered plagioclase and clinopyroxene microphenocrysts and about 5% plagioclase phenocrysts, cut by abundant veins of epidote and chlorite. Sample 109B is coarse-grained gabbro with large epidotized plagioclase crystals enclosed by chloritized-actinolitized clinopyroxene and minor olivine.

Chemistry The major and trace elements chemistry of 79 samples from the Burin Group suggest a geochemical evolution from basalts of alkalic affinity in the Pardy Island Formation upward to oceanic tholeiites in the remainder of the Group (Strong et al. 1978). In order to test and refine these observations, and to evaluate their petrogenetic significance, we have selected representative samples from each formation for study of the distribution of rare earth elements. The chemical data for these samples are presented in Table 1 and illustrated in Figs. 2 to 7. The samples described above were analyzed for REE, Sc and Co by instrumental neutron activation, with precision and accuracy as reported by Dostal and Capedri (1979). The variation in A1203/TiO2 ratios (Fig. 2) is mostly confined to the field of mid-ocean ridge basalts (MORB), although some samples of the Path End and Beaver Pond Formations extend into the field

of Betts Cove ophiolite, which Sun and Nesbitt (1978a, b) consider to have an island arc or inter-arc basin origin. Other classification diagrams involving Ti, Zr, and Y (Figs. 3 and 4) show a scatter concentrated mainly in the MORB field, while other diagrams (not presented) are equivocal, with some overlap between the MORB and island are tholeiite field for the Ti/Cr vs Ni diagram of Beccaluva et al. (1979), and most rocks in the low-K tholeiite field of the Ti vs Cr diagram of Pearce (1974). The latter classifications are also not consistent with the field relations and we regard these elements as not being immobile during greenschist facies alteration, an observation previously made (Strong 1977) for Phanerozoic basalts of Newfoundland. 16 14 12 I0 •

x

o

8

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2 •

o

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i

50

i

100 Zr

I

150

I

200

Fig. 3. Ti vs Zr (in ppm) compared to felds of mid-ocean ridge basalts (MORB), calc-alkaline basalts (CAB) and low potassium tholeiites (LKT) outlined by Pearce and Cann (1973). Symbols as for Fig. 2

Ti /I00 5060 ~ A R I A N A S ,~40 O

~/~k.~ ~ ~ J R _ETTS COVE i/~-4 ^

-,. 50 1 20

MORa

,o BARBERTON I

0 0

I

TiOz

9

,D

I

I

2

3

Fig. 2. Variation in AI203/TiO2 (wt.%) of the Burin Group compared with other mafic-ultramafic suites outlined by Sun and Nesbitt (1978 a, b). Abbreviations and symbols are as follows: STPK: spinifex-textured peridotitic komatiite; MORB: mid-ocean ridge basalt; dots: Pardy Island Formation; circles: Port au Bras Formation; filled triangles: Path End Formation; open triangles: Beaver Pond Formation; crosses." Wandsworth Gabbro

Zr

Yx3

Fig. 4. Variation in proportions of Zr, Ti and Y (ppm) in Burin rocks compared to fields outlined by Pearce and Cann (1973). Symbols as for Fig. 2

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Fig. 5a. Ratios of rare earth concentrations of the Pardy Island Formation normalized to chondrite concentrations given by Frey et al. (1968). The shaded field of alkali basalts includes those from Gough Island (Zielinski and Frey 1970), the Lesser Antilles (Schimuzu and Arculus 1975) and Hawaii (Schilling and Winchester 1969). 5 REE patterns of basalts from the Port au Bras (circles) and Path End (triangles) Formations compared to the field of common oceanic tholeiites (shaded) given by Frey et al. (1974). c REE patterns of basalts from the Beaver Pond Formation compared to some oceanic basaltic glass from the South Atlantic (dashes) given by Frey et al. (1974). d REE patterns of Wandsworth gabbros compared to oceanic gabbro field outlined by Frey et al. (1973)

The REE patterns are consistent with the classification and confirm the suggested geochemical evolution of the Burin Group. Figure 5a shows that patterns of the Par@ Island Formation with slightly enriched light rare earths (LREE) and slightly depleted heavy rare earths (HREE), are comparable to those of alkali basalts from a range of environments. Likewise, the two Port au Bras patterns and one Path End pattern are similar to those of oceanic tholeiites, with fiat REE patterns and chondrite ratios between 10 and 20 (Fig. 5b). The very low ratios of about 5 for the Path End sample T195 fall outside the oceanic tholeiite field, although the pattern remains flat. The Beaver Pond sample T390 (Fig. 5 c) is within the oceanic tholeiite field, but shows depletion of

LREE and slight enrichment of HREE. Sample T208 (Fig. 5 c) shows an even more extreme LREE depletion, and only those elements heavier than Tb fall within the oceanic tholeiite field. We note, however, that this pattern is very similar to that described by Frey et al. (1974) for basaltic glass of the Mid-Atlantic Ridge (Fig. 5c). The Wandsworth Gabbro REE patterns (Fig. 5 d) show a similar depletion in LREE to those of the Beaver Pond Formation and plot mostly within the oceanic gabbro field. However, they do not show a significant positive Eu anomaly, presumably indicating no significant plagioclase accumulation in these samples, although anorthositic bands are observed elsewhere.


170

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8

F )

cpx

4

E

cO

fp, O _ .40 (oL ~====~--,~-~

(~

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60 % 5

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GARNET LHERZOLITE

~I)

' ~

60%

PLAGIOCLASE LHERZOLITE

....

40

I xt_~T. 0.7

I

2

4

6

8

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Sm

Fig. 6. Models of rare earth variation during crystal fractionation or partial melting (after Pearce and Flower 1977). N u m b e r s refer to percent fractionation, accumulation or partial melting. For the partial melting models, trends (2-6) represent plagioclase lherzolite, and trend (1) represents a garnet lherzolite source. The shaded area represents Burin Group samples, with symbols as for Fig. 2

Petrogenesis

The major and trace element data described previously (Strong et al. 1978) show only a change from rocks of the Pardy Island Formation, to tholeiitic rocks of the four overlying formations, but do not reveal changes among the latter. The REE data, summarized on Fig. 7a, do show a distinct and progressive change from the oldest to the youngest unit within the Burin Group. From patterns with negative slopes of the Pardy Island Formation, the LREE are progressively depleted and HREE are enriched to give the flat patterns and ratios characteristic of oceanic tholeiites; the chondrite-normalized ratios of these flat patterns are lowered with higher stratigraphic level from the Port au Bras to the Path End Formation; the LREE are then progressively depleted and HREE enriched from the Beaver Pond Formation to the Wandsworth Gabbro. These systematic temporal changes in a cogenetic suite are the reverse of what one would expect from any crystal fractionation process involving differentiation of a parental magma. Although both LREE depletion and enrichment during low grade metamorphism of basalts have been reported (e.g., Frey et al. 1974; Wood et al. 1976), we dismiss any secondary

causes because of the improbability of their relating so closely to stratigraphic level (with no apparent petrographic evidence of systematic variations in secondary mineralogy). Rather in line with the model based on field and other evidence given by Strong et al. (1978), we suggest that these patterns can be best explained by some process involving progressive partial melting of the upper mantle. Pearce and Flower (1977) reviewed some petrogenetic variables governing magma genesis at accreting plate margins, and their calculated Ce/Sm behavior in both magma chamber and partial melting processes are shown in Fig. 6 (see caption for explanation). It is clear from this diagram that the trend of variation in the Burin Group Ce/Sm ratios is similar to that calculated for a single-stage partial melting process, although the ratios cover those suggested for their steady state factionating magma chamber model. It is clear that crystal-liquid fractionation, according to the steady state model, and using minerals and partition coefficients used by them, cannot provide an acceptable model for evolution of the Burin Group. One might suggest that a simple partial melting model would be adequate, but involving mantle with lower Sm and/or higher Ce than that assumed by Pearce and Flower. However, these values in the


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